Is there a tool for those days when you just can't mix the right shade of sky-blue? Introducing, the cyanometer. Created by Horace-Bénédict de Saussure, this circular color-value scale helps you to determine the blueness of the sky.
"But how to measure 'blueness'? Using suspensions of Prussian blue, Saussure dyed paper squares every shade of blue he could distinguish between white and black. These were assembled into a numbered colour circle that could be held up to the zenith at a standard distance from the eye - the matching square established the degree of blue" (Sella)